Date:   Tue, 1 Dec 2020 13:29:53 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 0/7] libbpf: add support for kernel module BTF
 CO-RE relocations

On Mon, Nov 30, 2020 at 7:49 PM Andrii Nakryiko <andrii@...nel.org> wrote:
>
> Implement libbpf support for performing CO-RE relocations against types in
> kernel module BTFs, in addition to existing vmlinux BTF support.
>
> This is a first step towards fully supporting kernel module BTFs. Subsequent
> patch sets will expand kernel and libbpf sides to allow using other
> BTF-powered capabilities (fentry/fexit, struct_ops, ksym externs, etc). For
> CO-RE relocations support, though, no extra kernel changes are necessary.
>
> This patch set also sets up a convenient and fully-controlled custom kernel
> module (called "bpf_testmod"), that is a predictable playground for all the
> BPF selftests, that rely on module BTFs.
>
> v2->v3:
>   - fix subtle uninitialized variable use in BTF ID iteration code;

While testing this patch I've hit this:
#111 test_bpffs:OK
[  222.418122] 9pnet_virtio: no channels available for device hostshare0
[  222.688972] sysfs: cannot create duplicate filename '/kernel/btf/bpf_preload'
[  222.689811] CPU: 0 PID: 2485 Comm: modprobe Tainted: G           O
    5.10.0-rc3-00874-gfe521960f79f #3078
[  222.690813] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.11.0-2.el7 04/01/2014
[  222.691681] Call Trace:
[  222.691937]  dump_stack+0x9a/0xcc
[  222.692303]  sysfs_warn_dup.cold+0x17/0x24
[  222.692714]  sysfs_add_file_mode_ns+0x38c/0x560
[  222.693200]  sysfs_create_bin_file+0xf8/0x150
[  222.693635]  ? sysfs_add_file_to_group+0x150/0x150
[  222.694140]  ? __kasan_kmalloc.constprop.0+0xc2/0xd0
[  222.694648]  btf_module_notify+0x6ac/0xd30
[  222.695111]  notifier_call_chain_robust+0xa6/0x1a0
[  222.695658]  blocking_notifier_call_chain_robust+0x61/0x90
[  222.696251]  load_module+0x62da/0x9f40
[  222.696627]  ? rcu_read_lock_sched_held+0x81/0xb0
[  222.697156]  ? module_frob_arch_sections+0x20/0x20
[  222.697632]  ? ima_read_file+0x140/0x140
[  222.698027]  ? security_kernel_post_read_file+0x68/0xb0
[  222.698643]  ? __do_sys_finit_module+0xf7/0x150
[  222.699126]  __do_sys_finit_module+0xf7/0x150
[  222.699588]  ? __ia32_sys_init_module+0xa0/0xa0
[  222.700049]  ? bpf_prog_32007c34f7726d29_bpf_prog1+0x198/0x7ec
[  222.700690]  ? rcu_read_lock_bh_held+0x90/0x90
[  222.701161]  ? __bpf_trace_sys_enter+0x4f/0x60
[  222.701601]  ? syscall_trace_enter.isra.0+0x174/0x230
[  222.702123]  do_syscall_64+0x2d/0x40
[  222.702483]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  222.703002] RIP: 0033:0x7fd909d727f9
[  222.703385] Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40
00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24
08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 57 76 2b 00 f7 d8 64 89
01 48
[  222.705258] RSP: 002b:00007ffeecdb8e28 EFLAGS: 00000206 ORIG_RAX:
0000000000000139
[  222.706010] RAX: ffffffffffffffda RBX: 0000000000625380 RCX: 00007fd909d727f9
[  222.706778] RDX: 0000000000000000 RSI: 000000000041a213 RDI: 0000000000000000
[  222.707522] RBP: 000000000041a213 R08: 0000000000000000 R09: 00000000006242a0
[  222.708241] R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
[  222.708932] R13: 0000000000625530 R14: 0000000000040000 R15: 0000000000625380
[  222.709715] failed to register module [bpf_preload] BTF in sysfs: -17
[  222.892778] sysfs: cannot create duplicate filename '/kernel/btf/bpf_testmod'
[  222.893609] CPU: 2 PID: 2493 Comm: test_progs Tainted: G
O      5.10.0-rc3-00874-gfe521960f79f #3078
[  222.894622] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.11.0-2.el7 04/01/2014
[  222.895498] Call Trace:
[  222.895768]  dump_stack+0x9a/0xcc
[  222.896128]  sysfs_warn_dup.cold+0x17/0x24
[  222.896573]  sysfs_add_file_mode_ns+0x38c/0x560
[  222.897047]  sysfs_create_bin_file+0xf8/0x150
[  222.897506]  ? sysfs_add_file_to_group+0x150/0x150
[  222.898004]  ? __kasan_kmalloc.constprop.0+0xc2/0xd0
[  222.898524]  btf_module_notify+0x6ac/0xd30
[  222.898968]  notifier_call_chain_robust+0xa6/0x1a0
[  222.899477]  blocking_notifier_call_chain_robust+0x61/0x90
[  222.900040]  load_module+0x62da/0x9f40
[  222.900434]  ? rcu_read_lock_sched_held+0x81/0xb0
[  222.900951]  ? module_frob_arch_sections+0x20/0x20
[  222.901474]  ? ima_read_file+0x140/0x140
[  222.901881]  ? security_kernel_post_read_file+0x68/0xb0
[  222.902450]  ? __do_sys_finit_module+0xf7/0x150
[  222.902937]  __do_sys_finit_module+0xf7/0x150
[  222.903401]  ? __ia32_sys_init_module+0xa0/0xa0
[  222.903867]  ? bpf_prog_32007c34f7726d29_bpf_prog1+0x198/0x7ec
[  222.904480]  ? rcu_read_lock_bh_held+0x90/0x90
[  222.904935]  ? __bpf_trace_sys_enter+0x4f/0x60
[  222.905401]  ? syscall_trace_enter.isra.0+0x174/0x230
[  222.905925]  do_syscall_64+0x2d/0x40
[  222.906304]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  222.906818] RIP: 0033:0x7fd58327e7f9
[  222.907193] Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40
00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24
08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 57 76 2b 00 f7 d8 64 89
01 48
[  222.909049] RSP: 002b:00007ffce1e96fe8 EFLAGS: 00000246 ORIG_RAX:
0000000000000139
[  222.909823] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fd58327e7f9
[  222.910541] RDX: 0000000000000000 RSI: 000000000165481f RDI: 0000000000000004
[  222.911260] RBP: 0000000000000004 R08: 00007ffce1e97230 R09: 0000000000000000
[  222.911995] R10: 0000000000002011 R11: 0000000000000246 R12: 0000000000000000
[  222.912710] R13: 00007ffce1e97230 R14: 0000000000000000 R15: 0000000000000000
[  222.913503] failed to register module [bpf_testmod] BTF in sysfs: -17

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